[CELEBORN-1099] Check register when handleGetReducerFileGroup

[onebox-li]

What changes were proposed in this pull request?

For spark case, when stage outputPartitioning is satisfied and no longer needs shuffle exchange, there will be no shuffle write procedure, same goes for RegisterShuffle, currently the origin reduce stage will throw a NPE when LifeCycleManager handleGetReducerFileGroup.

ERROR [dispatcher-event-loop-11] Inbox: Ignoring error
java.lang.NullPointerException: null
    at org.apache.celeborn.client.commit.ReducePartitionCommitHandler.handleGetReducerFileGroup(ReducePartitionCommitHandler.scala:307)
    at org.apache.celeborn.client.CommitManager.handleGetReducerFileGroup(CommitManager.scala:266)
    at org.apache.celeborn.client.LifecycleManager.org$apache$celeborn$client$LifecycleManager$$handleGetReducerFileGroup(LifecycleManager.scala:556)
    at org.apache.celeborn.client.LifecycleManager$$anonfun$receiveAndReply$1.applyOrElse(LifecycleManager.scala:298) 
    at org.apache.celeborn.common.rpc.netty.Inbox.$anonfun$process$1(Inbox.scala:115)
    at org.apache.celeborn.common.rpc.netty.Inbox.safelyCall(Inbox.scala:222)
    at org.apache.celeborn.common.rpc.netty.Inbox.process(Inbox.scala:110)
    at org.apache.celeborn.common.rpc.netty.Dispatcher$MessageLoop.run(Dispatcher.scala:227)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

Reproduce example like:
select count(*) as cnt from tableA;
And tableA is empty.

So here fix code to adapt to this normal situation. For Flink client, just follows the old behavior.

Why are the changes needed?

Fix the probable NPE.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Cluster test.

[codecov]

Codecov Report

Merging #2056 (f7ef1a3) into main (70366ed) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2056      +/-   ##
==========================================
- Coverage   46.79%   46.64%   -0.15%     
==========================================
  Files         165      165              
  Lines       10525    10525              
  Branches      957      957              
==========================================
- Hits         4924     4908      -16     
- Misses       5281     5294      +13     
- Partials      320      323       +3     

see 3 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[waitinfuture]

LGTM, thanks! Merging to main(v0.4.0)/0.3(v0.3.2)